The newlyweds who were the UK's first confirmed cases of swine flu believe they caught the virus on the flight home from Mexico, from five other men who were clearly ill.

The couple, Iain and Dawn Askham, from Polmont near Edinburgh, said they were sitting next to five men who were coughing and shivering on the Thomson First Choice flight 578, which landed in Birmingham on 21 April from the Mexican resort of Cancún.

"I actually said to Dawn, 'I think we're going to be getting off this plane with the plague,' " Askham told the Daily Record, adding that the couple were "99% sure" they contracted the virus on the flight.

The couple returned home last night after five nights in separate rooms at an isolation unit at Monklands hospital in Airdrie. They were diagnosed with swine flu last Saturday.

One other passenger on the flight, the 12-year-old girl from Paignton in Devon, has since been diagnosed with swine flu. Askham said at one point on the flight two other passengers sitting near the five British Asian men moved seats because of the disturbance from their coughing.

"We were on the flight back for nine hours and it would have been impossible not to have breathed in their germs, which will have been getting circulated around the plane," he said.

"They were really ill, especially the guy sat directly behind us. He had a really bad fever. We didn't ask to get moved because we'd have felt a bit rude asking for an upgrade, even if it was our honeymoon."

Their claims came as it emerged that the first Briton suspected of having caught swine flu within the UK is one of Iain Askham's best friends, a member of his six-a-side football team who fell ill after a night out with Askham last week.

Graeme Pacitti, 24, a clerical worker at Falkirk Royal hospital, had initially tested negative for flu but was retested after he began feeling unwell.

A new test brought a faint positive result for influenza A, the type which includes swine flu, leading health experts to believe he is the first "probable" swine flu victim to contract the virus within the UK.

His samples are now being tested again at the Health Protection Agency's laboratory at Colindale, north London. The result is expected to be announced later today by the Scottish health secretary, Nicola Sturgeon.

Officials at the Health Protection Agency said all the passengers on the flight have either been traced or are being tracked down. The agency said it contacted the airline soon after the Askhams were interviewed to discover who they had been close to since they became ill.

The agency was unable to confirm, however, whether the five men identified by the Askhams either had been traced or ruled out as suspected cases.

"Contact tracing has already been done," an agency spokeswoman said. "I can't tell you about individual cases because of patient confidentiality.

"What I can tell you is that everybody who was in close contact, the 1-metre area for more than an hour ‑ obviously anyone near them on a long-haul flight ‑ either has been or is being traced. That has been going on since the cases were identified as a positive."

The Scottish health authorities said 22 "close contacts" of the Askhams were identified and tested soon after the couple were admitted to Monklands hospital on Saturday night. One of those, Graeme Pacitti, has emerged as the only "probable" case of onward infection so far.

It is unclear, however, whether those 22 cases only involved people who were with the Askhams after they fell ill or whether they include passengers close to them on the Thomsons First Choice flight affected. Pacitti was among friends who went drinking with Askham on Thursday last week, the first day Askham had begun feeling unwell and two days after the couple's flight landed at Birmingham airport.

Pacitti has now been given the antiviral drug Tamiflu, and told to stay at home with his mother, Lesley, her partner, and his brother, Alan, 18. He told the Daily Telegraph: "It is just typical flu. I have a sore throat, a sore head and an upset stomach. I am just waiting to be re-tested again and hope to get more information later today."

Pacitti is the goalkeeper in Askham's six-a-side amateur football team FC Mallard. Three other team members, including Askham's best man at his wedding on 4 April, were among the first set of nine suspected cases within the 22 "close contacts" identified earlier this week.

By yesterday afternoon, the trio and nearly all other suspected cases had proven negative. The Scottish authorities have a further 21 new cases under investigation and six outstanding results from earlier this week.